Radioactivity. We all know about it, and it makes us horrifyingly ill when we receive an overdose.
IC2 nuclear explosions only have an immediate effect on whatever happens to wander into the blast radius at the time of the explosion, at the moment.
We can do much, MUCH better than this.
My thoughts - Radioactive Dust. Forms a thin layer of dust on a block, can be collected with a shovel like snowballs. However, going near it without a Hazmat suit is insanely dangerous, and leads to outright poisoning and possible death. Each radioactive dust block would have a radius of effect depending on how much more dust is adjacent to it, with a minimum of a 2 block reach away from itself.
In the event of a nuclear explosion, this dust is littered about the resulting crater following a similar algorithm to regular explosions (ie: raycasting to destroy blocks). After the initial explosion, just run a check on the explosion radius and have a random chance to plant a dust block on whatever block is hit by the raycast, with a greatly increased chance of it appearing on the tops of surfaces. Sides and bottoms could be coated in radioactive dust as well, but to a much lower degree.
This dust isn't just harmful, though. If processed correctly it can produce a fertilizer with an increased chance of causing a plant mutation (of course, with severely reduced GGR). It can also be used to refuel uranium cells, or be formed into an alternative to glowstone (albeit relatively dangerous).
The downside to this stuff? You have to blow something up to get it.